Nitride semiconductors such as GaN, AlN, and InN, or mixed crystals of these materials have wide band gaps, and are used as high-output electronic devices, short-wavelength-light emitting devices, and the like. For example, GaN as a nitride semiconductor has the band gap of 3.4 eV, which is greater than the band gap of 1.1 eV of Si and the band gap of 1.4 eV of GaAs.
As such high-output electronic devices, field-effect transistors (FET), high-electron-mobility transistors (HEMT), and the like have been available (see, for example, Patent Document 1). A HEMT that uses nitride semiconductors is used for a high-output, a high-efficiency amplifier, a high-power switching device, or the like. Specifically, in a HEMT that uses AlGaN in an electron supply layer and GaN in an electron transit layer, piezoelectric polarization or the like is generated in AlGaN due to distortion caused by different lattice constants between AlGaN and GaN, and high-density 2DEG (Two-Dimensional Electron Gas) is generated. Therefore, such a HEMT can operate at high-voltage, and can be used for a high-efficiency switching element, a high-voltage resistance electric power device for an electric vehicle and the like.
Incidentally, some devices using nitride semiconductors for super high frequency may use InAlN having high spontaneous polarization instead of AlGaN in an electron supply layer to implement a high-output device. InAlN can induce high-density, two-dimensional electron gas even if thinly formed, and hence, has drawn attention as a material that can realize both high output and high frequency. Such a HEMT has an insulation film of SiN or the like formed over the electron supply layer, as a passivation film.